The present invention relates to electronic devices formed in silicon carbide (SiC) and in particular relates to devices that incorporate insulating layers or substrates in or with silicon carbide.
Because of its favorable electronic properties, silicon carbide is theoretically well-suited for high power, high frequency devices, including by way of example and not limitation those that operate in the microwave and related frequency ranges.
Many such devices require the use of highly resistive substrates or layers in order to operate at such power levels and frequency ranges. Accordingly, a number of techniques have been developed to controllably produce silicon carbide with the desired properties.
As further background, the terms, “insulating,” “highly resistive,” “high resistivity,” and “semi-insulating,” tend to be used interchangeably in the context of certain types of silicon carbide-based devices. Accordingly, they will be used in that sense herein with any different uses of these terms being clear in context.
Some presently favored techniques for obtaining semi-insulating silicon carbide require either highly purified materials or precise doping control or both to create a desired and relatively exact compensation of dopants in the silicon carbide. As those of skill in this art are aware, compensating silicon carbide with both acceptor dopants and donor dopants, potentially also including intrinsic or other point defects, can produce the desired semi-insulating properties. These techniques for achieving high resistivity SiC can tend to be relatively complex, however, particularly at the high temperatures (typically in the range of 2000° C.) required to grow silicon carbide.
Other techniques for obtaining the necessary insulating characteristics incorporate oxide layers for this purpose. Oxide layers are somewhat attractive in silicon carbide because silicon carbide will oxidize to form silicon dioxide (SiO2) in a manner analogous to the oxidation of silicon to form silicon dioxide. Thus, thermal oxidation provides one available and familiar technique for obtaining insulating layers or portions from or with silicon carbide.
In some circumstances, however, oxidizing a silicon carbide layer is inappropriate, undesirable, or even unavailable. Such can be the case where the insulating portion or layer is, because of desired or necessary manufacturing techniques, or the structure of the device, buried within other silicon carbide portions. In such a case, oxygen can be included in a manner analogous to silicon-on-oxide and SIMOX technologies.
Silicon-on-oxide (also referred to as “silicon-on-insulator” or “SOI”) is well understood in the art and provides a technique for manufacturing oxide-gate transistors that are both smaller and can operate at higher frequencies than earlier generations of metal oxide semiconductor field effect transistors (MOSFETs or MOS) or complementary metal oxide semiconductor field effect transistors (typically referred to as CMOS).
Silicon-on-insulator places a transistor's silicon junction area on top of an electrical insulator, typically silicon dioxide. The SOI technique (and resulting structure) minimizes the capacitance of the gate area. By doing so, SOI minimizes gate capacitance, and thus reduces the charging and discharging time required by the MOS gate, which in turn permits faster transistor operation.
Silicon-on-insulator, however, usually requires placing a crystalline silicon layer (or analogously an SiC layer) on top of a generally amorphous silicon dioxide layer. For a number of reasons related to crystal growth, such a structural requirement presents greater technical complexity and less cost-effectiveness than is otherwise desirable for the resulting devices and circuits.
A potential solution was initially developed by IBM in the form of separation by implantation of oxygen, commonly referred to as SIMOX. The SIMOX process consists of two basic steps. In the first, oxygen ions are implanted into a wafer forming an insulating layer that separates two portions of silicon. A high-temperature anneal repairs implantation damage and produces the desired buried SiO2 insulating layer. Devices (e.g., transistors) are then built upon the top silicon layer. The technique produces high quality silicon-on-insulator wafers.
Although silicon-on-insulator in general and SIMOX in particular have their own advantages, adding oxygen to silicon carbide requires an implantation ratio of 2.00 (for SiO2) using atoms having an atomic mass of 16. Both of these factors can damage those portions of the silicon carbide through which the ions are implanted.
In turn, repairing implantation damage adds an additional manufacturing step, and if the damage is overly-extensive, it can simply render the wafer (or wafer portion) useless. Given the greater complexity of producing and handling silicon carbide as compared to silicon, the damage raised by implantation provides a similarly greater problem and technical hurdle.
As another issue, oxygen diffuses in silicon carbide. As a result, in silicon carbide-on-insulator structures, oxygen from the insulator layer can diffuse into the device layers. The diffused oxygen can create undesired oxides or otherwise interfere with the SiC layers and reduce or destroy their structural and functional characteristics and advantages.
As yet another problem, because many devices in silicon carbide are based upon epitaxial growth and resulting epitaxial layers on silicon carbide, damaging a substrate or base layer produces a damaged structure that must be cured or otherwise addressed before further satisfactory epitaxial growth can be carried out upon it.
Accordingly, techniques that can (or could) provide such buried insulating layers while avoiding these disadvantages offer corresponding advantages in silicon carbide device technology.